1. Field of the Invention
The present invention relates to an optical filter suitable for an optical transmission system or the like; and, in particular, to an optical filter in which a long-period grating in an optical waveguide such as an optical fiber causes, in light having a predetermined wavelength, a core mode to effect mode coupling with a cladding mode, thereby attenuating core mode light having the predetermined wavelength.
2. Related Background Art
Optical filters have transmission characteristics depending on the wavelength of light and can be used, for example, in an optical transmission system together with an optical amplifier, so as to equalize the wavelength dependence of gain of the optical amplifier. Namely, in an optical amplifier which optically amplifies incident light in a predetermined wavelength band and outputs thus amplified light, its gain upon optical amplification may vary depending on the wavelength. Also, the intensity of incident light may vary depending on its wavelength, thereby the intensity of the light outputted after being optically amplified may vary depending on the wavelength as well. An optical filter having such a transmission characteristic as to yield a loss peak wavelength at the same wavelength as the peak wavelength of the output light from the optical amplifier equalizes the wavelength dependence of the intensity of light outputted from the optical amplifier.
As such an optical filter, an optical fiber (optical fiber type optical filter) formed with a long-period grating in a predetermined area (filter area) has been known. This optical fiber type filter is preferably used due to its small insertion loss in optical fiber transmission systems or the like. In this type of optical filter, the long-period grating formed in the optical fiber causes a part of core mode light to effect mode coupling with cladding mode light, thereby attenuating a desirable light component. The transmission characteristic of the optical filter depends on the propagation constant of core mode light, the propagation constant of cladding mode light, the coupling constant between core mode light and cladding mode light, and the period of change in refractive index (grating period) in the long-period grating, and can be analytically obtained by solving mode coupling equations.